A new study could supply the missing link in the story of the brain consolidates floating memories into a permanent structure while we sleep. The study, published in the journal eNeuro, shows how a brain region, appropriately called the nucleus reuniens, ‘goes between’ two other memory-linked structures – the hippocampus and the prefrontal cortex.
As a result of this mediation, which occurs during slow wave sleep, the brain shows highly coordinated slow large pulsations in these two widely separated areas, which leads to establishing and organizing memories into a long-term form.
The nucleus reuniens may be responsible for coordinating activity between the prefrontal cortex and hippocampus, study shows. Credit: University of Alberta
Sleep and the brain
Sleep is a unique phenomenon in which the body fails to respond to conscious cues from other outside influences, but the brain acts in complete synchrony, harmonizing and consolidating various areas of functional learning and memory in its different regions. This is especially recognizable in the coordination that occurs between the hippocampus and the neocortex, and which underlies the establishment of episodic memory.
Slow-wave sleep is the stage at which sleep occurs in the deepest form. This type of sleep is critical to ensure recovery of muscle and brain health, and in the organization of memory.
The study and its findings
The current study was carried out in rats anesthetized with urethane, to simulate forebrain inactivity during sleep. At this time, the nucleus reuniens, an area within the thalamus, that is linked to both prefrontal cortex and hippocampus, shows a pattern of slow pulsatile activity within its neurons when the forebrain is inactive, which synchronizes with very slow but large brain oscillations, occurring at a rate of one a second.
When the neurons within the nucleus reuniens, or the fibers leading from these neurons to the cingulum bundle of nerves, were activated by optogenetics, a particular layer of nerve cells (called the stratum lacunosum-moleculare, or SLM) within the hippocampus responded by producing nerve impulses. Interestingly, the SLM is the same layer of neurons in which the power of slow-wave sleep-related slow oscillations is maximum. Similar findings occurred when the medial part of the prefrontal cortex was stimulated, though with a slightly longer delay.
Next, they inhibited the neurons in the nucleus reuniens so that they could not transmit impulses. They then repeated the earlier stimulation of the prefrontal cortex, which failed to evoke the initial hippocampal response. In addition, slow oscillations in the cortex and hippocampus were also shown to be thrown out of synchrony when the nucleus reuniens was inactivated. This shows that slow-wave activity in both these regions can only be coupled if the nucleus reuniens is active. In other words, this nucleus plays a key role in achieving coherence in slow oscillations across these regions.
The scientists conclude that this activity of the nucleus reuniens may be essential to the related slow wave sleep-related consolidation of episodic memory.
Earlier studies have shown that during brain active states, rapid constant nerve impulses are produced by the neurons of the nucleus reuniens, without obvious rhythmic or oscillatory characteristics. When the brain changes over to a deactivated state, and especially when slow oscillations occur in the forebrain, the nucleus reuniens neurons too show slow rhythmic activity patterns, synchronizing with the slow waves. This could mean that these neurons process information in different ways depending on the brain state, and also that they plays a role in tracking slow oscillation rhythms. In fact, they may perhaps transmit it from the prefrontal cortex to the hippocampus. The SLM is likely to be involved in this activity as it receives the strongest inputs.
Other studies also show that the nucleus reuniens is involved in the coupling of slow oscillations between these two brain areas, and vice versa, which serves to strengthen synaptic activity between nerve cells in two regions, both of which take part in episodic memory. This could well be how slow oscillations strengthen episodic memory retention and consolidation.
The nucleus reuniens could thus be an information hub controlling the flow of signals between these two regions, and its role in many memory tasks is becoming more widely recognized. When injured, it affects memory tasks in which both areas are required, rather than those which depend on the hippocampus alone. other researchers have also suggested that it plays a role in waking memory as well.
Researcher Brandon Hauer says, “Before this study, we did not know what was responsible for connecting the prefrontal cortex and the hippocampus. This under-studied and relatively unknown brain area likely has a substantial role in forming long-term memories while you sleep.”
The reuniens nucleus of the thalamus has an essential role in coordinating slow wave activity between neocortex and hippocampus. Brandon E. Hauer, Silvia Pagliardini and Clayton T. Dickson. eNeuro. doi: 10.1523/ENEURO.0365-19.2019. https://www.eneuro.org/content/6/5/ENEURO.0365-19.2019.